Temperature Converter (Celsius, Fahrenheit, Kelvin, Rankine)
Type a temperature, choose its scale, and see the equivalent value in the other three scales instantly. Uses exact conversion formulas, no rounding errors.
How it works
How the conversion formulas work
All four scales pivot through Celsius internally. Fahrenheit ↔ Celsius is F = C × 9/5 + 32 and C = (F − 32) × 5/9. Kelvin shifts Celsius by 273.15 (the absolute zero offset): K = C + 273.15. Rankine is to Fahrenheit what Kelvin is to Celsius — same scale spacing as Fahrenheit, but anchored at absolute zero: R = (C + 273.15) × 9/5.
These formulas are exact: the constants 273.15 (Kelvin offset) and 9/5 (Fahrenheit ratio) are defined by the SI system, not measured. Any rounding you see in the result is from converting to decimal display, not from the calculation itself.
When you'd use each scale
Celsius is the everyday scale almost everywhere except the United States. Water freezes at 0°C and boils at 100°C at sea level — clean, easy boundaries.
Fahrenheit, used primarily in the US, has a finer-grained 'feel' for ambient air temperatures: 0°F is very cold, 100°F is very hot, and most weather sits in the 30-90°F band. The scale was designed around brine and human body temperature in the 18th century.
Kelvin is the scientific standard. It removes the negative-number awkwardness because absolute zero is 0 K, not −273.15. Used in physics, chemistry, and astronomy. Rankine is the imperial-system equivalent of Kelvin — same purpose, but with Fahrenheit-sized degrees. It's still used in some US engineering contexts.
Useful reference points
Absolute zero: 0 K = −273.15°C = −459.67°F. Water freezes: 0°C = 32°F = 273.15 K. Body temperature: 37°C ≈ 98.6°F = 310.15 K. Water boils (sea level): 100°C = 212°F = 373.15 K. Hot oven: 200°C = 392°F. Earth's average surface temperature: about 15°C = 59°F = 288 K.
The Celsius and Fahrenheit scales meet at −40°: that's the same temperature on both. It's the only point where the two scales agree.
Frequently asked questions
›Why are these formulas exact?
The Celsius-Kelvin offset (273.15) and the Celsius-Fahrenheit ratio (9/5) are SI definitions. Any apparent imprecision is just decimal display rounding.
›Why does Kelvin not use a degree symbol?
By SI convention, Kelvin is written without °. So '300 K' is correct; '300°K' is not.
›What's the difference between Kelvin and Rankine?
Both are absolute scales (zero at absolute zero). Kelvin uses Celsius-sized degrees; Rankine uses Fahrenheit-sized degrees. 1 K = 1.8 R.
›What's the temperature where Celsius and Fahrenheit are equal?
−40°. It's the only crossover point: −40°C = −40°F.
›Why is body temperature 37°C and not 36 or 38?
37°C (98.6°F) is the historical average, but normal body temperature varies by 0.5°C through the day and from person to person. Modern data suggests the average is closer to 36.6°C.
›Can I convert temperature differences?
Be careful: a difference of 10°C equals 18°F (you don't add 32). For differences, multiply by 9/5 to go C→F or by 5/9 to go F→C, and skip the offset.
›Is the calculator accurate enough for science?
We display up to four decimals from double-precision math. For metrology-grade work, refer directly to NIST conversion tables, but for chemistry, cooking, and engineering this is more than sufficient.
›Why include Rankine if it's rarely used?
It still appears in some US thermodynamics texts and HVAC engineering work. Including it costs nothing and helps people who actually need it.
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